Accumulated Other Comprehensive Income (Loss) - Schedule of AOCI (Detail) - USD ($) $ in Thousands	12 Months Ended
	Dec. 31, 2020	Dec. 31, 2019	Dec. 31, 2018
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	$ 2,852,453	$ 2,689,570	$ 2,154,397
Amounts reclassified from AOCI to retained earnings related to the Tax Cuts and Jobs Act of 2017			(10,751)
Other comprehensive income (loss) before reclassifications	103,826	100,973	15,852
Amounts reclassified from AOCI to income	(12,262)	184	273
Ending Balance	2,972,656	2,852,453	2,689,570
Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(31,150)	6,322	(4,179)
Ending Balance		(31,150)	6,322
Accumulated Other Comprehensive Income (Loss)
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	56,207	(44,950)	(50,272)
Amounts reclassified from AOCI to retained earnings related to the Tax Cuts and Jobs Act of 2017			(10,751)
Ending Balance	147,771	56,207	(44,950)
Accumulated Other Comprehensive Income (Loss) | Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(52)
Unrealized Gains and Losses on Available- for-Sale Debt Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	56,131	(37,348)	(35,557)
Amounts reclassified from AOCI to retained earnings related to the Tax Cuts and Jobs Act of 2017			(7,583)
Other comprehensive income (loss) before reclassifications	97,596	94,964	7,454
Amounts reclassified from AOCI to income	(8,392)	(1,485)	(1,662)
Ending Balance	145,335	56,131	(37,348)
Unrealized Gains and Losses on Available- for-Sale Debt Securities | Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			0
Unrealized Gains and Losses on Held-to- Maturity Securities
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	0	(8,515)	(12,107)
Amounts reclassified from AOCI to retained earnings related to the Tax Cuts and Jobs Act of 2017			(2,600)
Other comprehensive income (loss) before reclassifications	0	6,419	4,514
Amounts reclassified from AOCI to income	0	2,096	1,678
Ending Balance	0	0	(8,515)
Unrealized Gains and Losses on Held-to- Maturity Securities | Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			0
Gains and Losses on Cash Flow Hedges
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	240	1,099	(2,337)
Amounts reclassified from AOCI to retained earnings related to the Tax Cuts and Jobs Act of 2017			(509)
Other comprehensive income (loss) before reclassifications	6,230	(410)	3,884
Amounts reclassified from AOCI to income	(3,886)	(449)	113
Ending Balance	2,584	240	1,099
Gains and Losses on Cash Flow Hedges | Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			(52)
Defined Benefit Pension Plans
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance	(164)	(186)	(271)
Amounts reclassified from AOCI to retained earnings related to the Tax Cuts and Jobs Act of 2017			(59)
Other comprehensive income (loss) before reclassifications	0	0	0
Amounts reclassified from AOCI to income	16	22	144
Ending Balance	$ (148)	$ (164)	(186)
Defined Benefit Pension Plans | Adjustment
Accumulated Other Comprehensive Income (Loss) [Line Items]
Beginning Balance			$ 0